Your ride will be a lot easier with the correct size of bike. Mountain bikes tend to be measured in inches or general size (Large, Medium, etc.), so it is handy to know your size in all variants if you are looking at multiple types of bike.
If you are between sizes, it usually works better if you go for the larger size and shorten the stem.
Mountain bikes come in a lot of sizes, each with their own advanages. Larger wheels do better on rougher ground while smaller wheels are quicker, lighter and stronger.
26” wheels are a good option for junior riders, small adults and jump bikes as they are easier to handle than larger wheels.
27.5” wheels are the most common wheels and are ideal for quick, balanced riding.
27.5”+ wheels give you more grip and cushioning (float) but overall make you a little heavier and slower.
29” wheels are more popular for cross-country, enduro and downhill because, though they are slower getting up to top speed, they carry speed much better over rough terrain, and offer much more grip than smaller wheels.